The influence of physical and chemical surface properties of silica on surface retention is described. The surface area within the column governs retention. The in- f luence of sodium content at concentrationsbelowO.1 % on selectivity is demonstrated. For characterization of reversed phases two different approaches are described. Asystematic evaluation of hydrophobic properties leads to four parameters sufficiently describing retention. A pragmatical test procedure including basic solutes permits with a single injectionto evaluate stationary phases for hydrophobicity and for their suitability for separation of basic solutes. The achievable selectivity of stationary phases for chromatography depends on their physical properties like specific surface area, pore diameter and porosity. More important than the usually available specific parameters is the knowledge of the surface area available within the column. To get these values the packing density is required, which one can get only by unpacking the column. Besides, by these commonly available parameters, chromatogra- phic selectivity is governed additionally by the surface chemistry of the stationary phase. For pure silica,selectivity is determined by the concen- tration of the surface silanols (ref. 1). One can differentiate at least between two surface groups (ref. 2), the isolated silanols, and those which are hydrogen-bonded to adjacent ones. These differences can be seen by DRIFT (diffuse-refeltctance infrared fourier tfansform spectroscopy) spec- tra. The sharp absorption band at 3740 cm- has been assigned to the isolated surface silanol groups, whereas hydrogen yonded groups give rise to a broad absorption band between 2500 and 3600 cm- . By thermal treatment the ratio of free to bonded silanol groups can be altered (ref. 3). Prima- rily at temperatures above 2OO0C the vicinal, hydrogen-bonded silanol groups are removed. For silicas treated at temperatures above 5OO0C vicinal surface silanols are no longer noticeable in the DRIFT-spectra. In Fig. 1 these spectras are compared to the retention behaviour of solutes which can interferewith hydrogen bonds, like phenol and alcohols and of solutes, which act solely as hydrogen-bond acceptors, like acetophenone and benzoic acid esters (ref. 4). It can be seen, that the retention of solutes containing hydroxyl groups decreases with increasing temperature treatment of the stationary phase, and hence decreasing amount of hydrogen-bonded surface silanols. The decrease in retention of these solutes is largest in the temperature range between 2OO0C and 60OoC. This indicates that these solutes are mainly adsorbed by breaking into the hydrogen bonds of the vicinal silanols. In contrast, the retention of solutes containing accep- tors for hydrogen bonds is hardly influenced by thermal treatment. This indicates that these molecules with "basic" properties are adsorbed prefe- rentially on the acidic isolated silanols, the surface concentration of which is not affected in this temperature range. Pure silica should show in aqueous suspension an pH value around 3 to 4. It has been demonstrated (ref. S), that commercially available silicas cover the pH range between 3.5 and 9.5. The high pH values could be correlatedwith the sodium content of the silicas. It is easily understandable that the retention of basic and acidic solutes is strongly affected by a sodium content below 0.15 8. In Fig. 2 a the separation of phenols and in Fig. 2 b that of anilines is demonstrated on different silica gels, where the pH had been adjusted either by an acid wash or by titration with NaOH. As eluent dry dichloromethane has been used. Besides retention also peak shapes show a distinct dependency on surface pH. Consequently, the acidic phenols are